Platelet monoamine oxidase (MAO), plasma amine oxidase (PAO) and serum Dopamine-B-Hydroxylase (DBH) activity is determined in all patients admitted to the Illinois State Psychiatric Institute site of the University of Chicago Mental Health Clinical Research Center (approximately 95 per year) and in normal controls. Platelet MAO activity was found to be significantly lower in hallucinating than in non-hallucinating schizophrenics diagnosed according to the Research Diagnostic Criteria or by Taylor and Abrams criteria but not by other systems. Hallucinating psychotic patients who are not schizophrenic do not have decreased platelet MAO activity. It was found that chronic treatment with neuroleptics can decrease platelet MAO activity. Some schizophrenic patients were found to have relatively high amounts of an inhibitor of MAO in serum. Platelet-rich-plasma was isolated from psychotic patients at two different centrifugation conditions. The variance in the ratio of the activities under these conditions was significantly greater in the patients, indicating that centrifugation conditions can affect the magnitude of the difference in the platelet MAO levels between patients and controls. PAO activity did not differ in any of the major psychiatric group studies. We have not found any evidence for an abnormality of the circadian rhythm in serum DBH activity in depressed patients but continue to find decreased serum DBH activity in our sample of depressed patients despite the fact that the patient sample studied this year does not show this effect. No serum DBH inhibitors were found in the serum of depressed patients.